Zone focus: a good friend in 'tough' autofocus situations

Saturday 27 October 2018, 14:41 by Huub Keulers | 9,073 views | 4 comments

This article was translated from Dutch with the help of AI. Read the original.

We have become so used to the autofocus (AF) on our lenses that we sometimes forget there are other ways to get your main subject sharp in your photos. One of these methods is called zone focus, also known as pre-focus.

Zone focus has traditionally been used a lot in street photography, mainly so you can take photos quickly and inconspicuously. However, this technique can also work well in landscape, sports and action photography.

In this article I will explain what zone focus is and when and how you can use this method. This is how I work, but I realise there are several methods that can produce the desired result.

A typical example of the zone focus method. Photographing the freestyle BMX rider from this angle using autofocus would have been more difficult. Now it was simply a matter of waiting until he entered the focus area of my camera and then taking the photo

What is zone focus

In sports and street photography, zone focus works on the principle that your moving main subject, which you want to show sharply in the photo, passes a point that you have focused on beforehand. If you take the photo at that moment, it can hardly go wrong and your main subject will be sharp, depending of course on the shutter speed you have chosen.

It is important that you switch off the autofocus on your camera or lens after focusing. Otherwise, your camera will focus again when you press the shutter button.

The zone focus method is used most often with the shorter focal lengths, wide angles from 12 to 35mm. These focal lengths give you a considerable depth of field, which means there is a good chance that your subject will be within this depth-of-field area and will therefore be sharp.

As you probably know, there is always one point in your photo that is at its sharpest. This is the point you focused on, leaving aside any back or front focus of your lens. From this point, the sharpness in your photo decreases towards the front and back, until the sharpness has completely disappeared to our eyes.

Another term for depth of field is DOF, Depth of Field. Depth of field is therefore the distance in metres between the sharpest point furthest to the front in your photo and the point furthest to the back.

The part of your photo that we consider to be sharp is called the depth-of-field area. This area is then 'in focus'
You always have less depth of field in front of your focus point than behind it. The ratio of this depth of field is roughly 1/3 to 2/3. This means you have more sharpness in metres behind your focus point than in front of it. So if you want a lot of depth of field in your photo, the point you focus on is very important.

Ratio in the depth-of-field area
When it comes to depth of field and using zone focus, there are a few rules you need to know in advance:

The shorter your focal length, meaning the wider the angle, the larger your depth-of-field area will be. With a telephoto lens, from 100mm onwards, you will therefore have a reasonably small depth-of-field area;

A small aperture opening, with a high aperture number, such as F11 or F16, gives you more depth of field than a large aperture opening, such as F2.8 or F4;

The smaller the sensor in your camera, the larger your depth-of-field area will be. Crop-sensor cameras therefore create greater depth of field than full-frame cameras.

In every situation there will always be one focus point where you have the greatest possible depth of field. This focus point is also called the hyperfocal distance. Since nobody knows this from memory, there are useful tools to help you find out how large your depth-of-field area is.

Examples include specific apps for your phone, such as Hyperfocal Pro for Android and Simple Dof Calculator for Apple's iOS. There are also tables that you can print out and keep in your camera bag.

My colleague Bas Meelker has made some useful tables for full-frame camera sensors and crop-sensor cameras. If you focus at the desired hyperfocal distance, the sharpness in your photo runs from 50% of this distance to infinity. So in the example of a 24mm focal length and aperture F11, your sharpness runs from 0.85 metres to infinity.

For use with full-frame camera sensors. Source: Bas Meelker

For use with crop camera sensors. Source: Bas Meelker

When do you use zone focus

To use zone focus properly, you do need to have some feel for distances. Not everyone is equally good at this, by the way. You may be surprised at how many people struggle if you ask them to focus at 3 metres in front of them.

You can actually use zone focus at any time, but it is especially useful when your moving subject follows a known route or when your autofocus is too slow to focus. Think of fast sports or running dogs.

If your 'subject' also has little contrast, such as an even colour without detail, AF may sometimes struggle and zone focus is a possible alternative.

You can also use zone focus when your subject suddenly appears in the frame and you have no time at all to focus, but you do know where your subject will pass. Or when there are obstacles between you and your subject and you know where your subject will pass.

Here are some examples of these situations.

Important: Make sure you stay in the same place after focusing in advance on a specific point. If you do not, you will change the distance between you and your subject, which may put it outside the depth-of-field area and mean it will not be sharp.

Street scene in San Remo. Focus distance 2.5 metres. Aperture F8

Focused in advance on the post, red circle

Focused in advance on the water bowl, red circle

Focused in advance on the track where the runners would pass, yellow circle

Focused in advance on the track, red circle

How do you use zone focus

Stand in a place where you get a good composition in your photo;

Set your aperture value. With moving subjects, make sure your shutter speed is fast enough to freeze the action;

Focus on the desired focus point. You can do this with your autofocus, but make sure you switch it off afterwards. People who use the Back Button Focus method do not need to do this. Their AF is already disconnected from the shutter button;

Another method is to set the hyperfocal distance manually on your lens. Your lens must have a distance scale for this;

Take your photos when your subject passes your focus point or is within the depth-of-field area;

Check your photos for sharpness and composition and adjust your focus point or camera settings if necessary.

At aperture F16 and focused at infinity, the depth of field with this 35mm lens runs from approximately 2.5 metres to infinity
I always explain zone focus to my students during a street or sports photography workshop. They are often surprised by how simple and reliable this method can be.

Give it a try in practice and let me know what you thought of it and whether it is useful to you.
Good luck and have fun taking great, in-focus photos.

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About the author

Huub Keulers is een enthousiasmerende veelzijdige freelance fotograaf woonachtig in Elsloo (Li). Hij is gespecialiseerd in sport-, bedrijfs-, straat-, honden en portretfotografie. Daarnaast is Huub auteur van diverse educatieve fotografieboeken en verzorgt hij allerlei fotoworkshops, masterclasses en lezingen.

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4 comments

Hugo VdV wrote on Saturday 27 October 2018 at 19:28 | reply
An addition to "rule 1":
That of course only applies when the distance between camera and subject is the same.
At the same magnification, the depth of field is the same, regardless of the focal length of the lens.
Translated from Dutch.

jkopprno1970@hotmail.com wrote on Saturday 27 October 2018 at 23:40 | reply
An article to keep, Huub. Thanks for it! Ever since the course I took with you, I’ve been using the BBF. Still to my complete satisfaction. And that despite the fact that I generally don’t photograph moving subjects. Your (and Marcel’s) book on Sports Photography is still on my list too! I still don’t have the time to study it just yet. Thanks again!
Translated from Dutch.

Huub Keulers wrote on Sunday 28 October 2018 at 11:44 | reply
Dear Hugo, you're absolutely right about law 1. Good addition. Thanks for your comment.
Translated from Dutch.

Huub Keulers wrote on Sunday 28 October 2018 at 11:47 | reply
Hi Jacqueline, you're very welcome and thank you for the lovely compliment. Your photos of non-moving subjects are also a pleasure to look at.
Translated from Dutch.

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